Cathode ray tube (CRT) screens are projection devices typically comprising an electron gun, circuitry to control the path of the electrons emitted, and a phosphor coated screen that glows after an electron strikes the screen. While CRT's have been a standard display projection device for many years, certain fundamental and characteristically inherent features of the CRT present problems and limitations.
The typical CRT screen is heavy and difficult to move, and thus dictates to the viewer his viewing position. Once in place, CRT screens including most televisions usually remain where initially positioned due to weight and maneuverability restrictions. While problems caused by the weight and bulk of CRT screens can be remedied in some instances by the use of a smaller CRT, using a smaller CRT also decreases the screen size. CRT screens are limited in size due to the requirement that the electrons must travel in a vacuum. Much of the weight and bulk of a CRT is due to the heavy walls containing the vacuum. As the size of the screen increases, the walls necessarily become heavier and thicker. There comes a point where it is not economically or mechanically feasible to make a large CRT screen.
CRT screens also emit radiation from electron acceleration and deceleration. Electron acceleration occurs at the electron gun where the electrons are emitted. Deceleration occurs at the phosphor coated screen where the electrons collide with the screen. While some of this radiation can be shielded from the viewer, often this shielding is not totally effective. This radiation may cause eye fatigue and strain after extended viewing of the CRT screen.
In an attempt to solve the limitations and problems of CRT screens, liquid crystal display (LCD) screens have been used with limited success. A liquid crystal is an electronic device that alters its ability to pass light when a voltage is applied. In an LCD screen, liquid crystals are arranged to form a matrix allowing particular selection of each liquid crystal so that light and dark areas may be formed on the screen. In one typical use, characters such as letters or numbers are represented by darkening appropriate liquid crystals, preventing light from behind those liquid crystals from reaching the viewer.
Many LCD screens, however, do not present clear images as the contrast between light and dark regions is not remarkably distinct. Further, LCD screens do not display images in color, limiting their use as entertainment and graphic display devices. In comparison with other projection devices, LCD screens have been consistently slower in changing from one image to another. This has prevented LCD screens from acting as projection display devices of sequential images such as the ones in usual television transmissions. Accordingly, while LCD screens avoid most of the problems associated with CRT screens, LCD screens do not project sequential images as do typical CRT screens.
Lightweight, non-radiating screens composed of a bundle of fiber optic cables viewed on end have been used to overcome the disadvantages of LCD and CRT screens. In such a screen, fiber optic cables are used to conduct light from a projection system to the screen so that the images presented may be viewed. A crude way of doing this is to have one end of the organized fiber optic cable bundle pressed next to a CRT screen so that the CRT image illuminates the bundle, transmitting the image. The other end of the fiber optic cable bundle is then fixed for viewing so that the viewer sees the CRT projection. In this case, the fiber optic cable bundle acts as an extension for the CRT. As fiber optic cables do not transmit the radiation associated with CRTs, eye strain from extended viewing is avoided. A fiber optic cable screen is lighter in weight and more maneuverable, although in a crude configuration a large heavy projection system may be present while the fiber optic cable screen is limited in maneuverability by its umbilical attachment to the CRT.
For the normal television signal composed of 525 raster rows each composed of 380 segments, 199,500 individual fiber optic cables are required for one to one representation of a video signal. This large number of individual fiber optic cables tends to make a fiber optic cable screen hard to handle. Furthermore, if the fiber optic cable screen is significantly larger than the size of the originally received image, the resulting image becomes grainy and less pleasing for a viewer.
Large screen projection televisions are another means for projecting or displaying images. With large screen projection televisions, colored light is projected onto a viewing screen. As with CRT screens, the large screen projection television is a bulky device. The screen itself is large and must be precisely placed in relation to its projector. Many large screen projection televisions have the large screen attached to the projector. Additionally, large screen televisions dictate the viewer's position in relation to the screen as the screen projects a clear and pleasing image to a restricted viewing area. Outside this area, the image is distorted and blurred.
In an attempt to overcome these problems, viewing systems have been developed that are light, maneuverable, and which emit no irritating radiation. An example is shown in U.S. Pat. No. 4,578,709. Although such a flat optical television screen has provided significant advancements over typical CRT screens, additional features and useful modifications are desirable in order to provide a more commercially acceptable and technically superior product.
There exists, therefore a need for a visual display that overcomes the disadvantages of bulk, weight and unmaneuverability. Additionally, a need exists for a display system that emits no eye-fatiguing radiation. These disadvantages should be eliminated while preserving the advantageous viewing characteristics of previous projection screens. Further, a need exists for an improved projection screen that uses a combination of light emitting sources and light controlling shutters to present an image. Such a system would preferably be of simplified construction and maximize the size of the projected image with respect to the overall physical housing for the viewing system. The present invention fulfills these needs and provides further related advantages.